The present invention relates to methods and apparatus for coding and decoding moving pictures.
Moving pictures are transmitted in digital form in a variety of systems, which will soon include digital television broadcasting systems. To reduce the quantity of transmitted data, digitized moving pictures must be coded at the transmitter and decoded at the receiver. The MPEG-2 coding system standardized by the Moving Picture Experts Group is widely used. In this system, a moving picture is coded predictively by a series of steps involving motion estimation, motion compensation, a discrete cosine transform (DCT), quantization, and variable-length coding. The coded data are decoded by a reverse process involving variable-length decoding, inverse quantization or dequantization, an inverse discrete cosine transform (IDCT), and motion compensation.
A general problem in this and other similar coding systems is their high computational cost. A significant part of the cost occurs in the IDCT, which involves many multiplication operations, the efficient performance of which requires extra hardware and software. Another cost occurs because of the large amount of semiconductor memory required for storing the reference pictures used in motion compensation. The many accesses required to this memory are yet another computational cost. These costs are particularly great when the transmitted picture is a high-definition television (HDTV) picture.
It would obviously be desirable to reduce these costs, particularly in broadcast receivers, the high price of which hinders the acceptance of digital broadcasting.
It would also be desirable to enable HDTV transmissions to be received by receivers equipped with standard-definition display screens, which are less expensive than high-definition display screens. This requires a process of resolution conversion.